Water-absorbing polymeric particles (also referred to as superabsorbent polymer particles or superabsorbers) are widely used in absorbent articles. These are materials that are able to take up and retain several times their weight in water, possibly up to several hundred times their weight, even under moderate pressure. Absorbing capacity is usually lower for salt-containing solutions compared to distilled or otherwise de-ionised water. Typically, a superabsorbent has a centrifugal retention capacity (“CRC”, as measured by the CRC method set put herein) of at least 5 g/g, preferably at least 10 g/g and more preferably at least 15 g/g. To improve their performance characteristics these water-absorbing polymeric particles are generally post-crosslinked, e.g. with organic crosslinkers. This post-crosslinking is preferably carried out as a surface post-crosslinking with ground and classified base polymeric particles.
The acrylate-based superabsorbents which dominate the market are produced by radical polymerisation of acrylic acid in the presence of a crosslinking agent (the “internal crosslinker”), usually in the presence of water, the acrylic acid being neutralized to some degree in a neutralisation step conducted prior to or after polymerisation, or optionally partly prior to and partly after polymerisation, usually by adding a alkali, most often an aqueous sodium hydroxide solution. This yields a polymer gel which is comminuted (depending on the type of reactor used, comminution may be conducted concurrently with polymerisation) and dried. Usually, the dried powder thus produced (the “base polymer”) is surface crosslinked (also termed surface “post”crosslinked, or just “postcrosslinked”) by adding further organic crosslinker to generate a surface layer which is crosslinked to a higher degree than the particle bulk. A polyvalent metal cation may also (alternatively or in addition) be used, such as aluminum sulphate; applying polyvalent metal cations to superabsorbent particles is regarded as “surface complexing” or as another form of surface treatment, although it has also the effect of increasing the number of bonds between individual polymer strands at the particle surface and thus increases gel particle stiffness as organic surface crosslinkers have. Organic and polyvalent metal surface crosslinkers can be cumulatively applied, jointly or in any sequence.
Surface crosslinking leads to a higher crosslinking density close to the surface of each superabsorbent particle. Although surface crosslinking decreases the CRC or other parameters describing the total absorption capacity of a superabsorbent sample, it addresses the problem of “gel blocking”, and it can increase the permeability of the gel bed (measured in SFC value, as described herein); overall, the surface cross-linking may well increase the total amount of liquid that can be absorbed by a hygiene product containing a given amount of superabsorbent during normal use of the product.
There is still a need to provide even thinner absorbent articles since they increase the wearing comfort. There has been a trend to remove part or all of the absorbent cellulose fibers (pulp) from the products. Such ultrathin hygiene articles, for example diapers, may have absorbent cores that primarily comprise such water-absorbent polymeric particles, e.g. from 50% to 100% by weight of the absorbent materials in said absorbent cores. In such absorbent cores, the water-absorbing polymeric particles not only perform the storage function for the fluid but also ensure active fluid transportation (wicking absorption) and passive fluid transportation. The greater the proportion of cellulose pulp which is replaced by water-absorbing polymeric particles, the greater the number of transportation functions which the water-absorbing polymeric particles have to perform in addition to their storage function. It has been found that for such absorbent articles in particular, there is a need for water-absorbent polymeric particles that have a good absorbent capacity (CRC value) and a good fluid transportation (reflected by a good FHA value and SFC value). It is well-known in the art that there is a trade-off between the permeability and capacity/absorbency.
The need for an improved SFC has been known and examples of improved water-absorbent polymeric particles are for example described in WO 2005/014064, which for example teaches to coat a water-absorbent polymer with an elastic film-forming material.
The traditionally used absorbent cellulose fibers (pulp) serve, in addition to providing liquid absorption and transport, also to fix the water-absorbent polymeric particles. In the absence of such pulp, other fixation aids, such as fibrous thermoplastic material and/or adhesive material is used in the absorbent cores, to fix or stabilize the water-absorbent polymeric particles in the absorbent structure.
It has been noticed that this type of fixation requires a sufficiently high wicking absorption (FHA) at least in the storage layer due to the fact cellulose fibers are either not present or used in very small amounts in these novel absorbent composite structures.
The present invention therefore has for its object to provide absorbent structures, for use in hygiene articles, such as diapers and feminine hygiene articles, comprising water-absorbing polymeric particles having a good fluid transportation and preferably sufficiently high initial uptake rates, whilst having a good absorbent capacity, and in particular, such absorbents structures that comprise little or no pulp (or absorbents cellulose fibers), and that typically comprise other immobilization/structuring aids such as thermoplastic material and or adhesive materials, optionally in the form of (non-absorbing) fibers.